1. Field of the Invention
The present invention relates to the field of cryogenic detectors and in particular to the means employed within infrared cryogenic detectors to deal with the thermal effect of liquefied air in contact with the cold finger of the detector.
2. Description of the Prior Art
Infrared detectors, used in optical guidance systems, operate at cryogenic temperatures at or below the boiling point of liquid nitrogen, i.e., approximately 77 degrees Kelvin. The infrared detector is cooled by means of a cold finger within a dewar. As the entire apparatus cools from ambient temperatures, gases in the air within the dewar liquefy. Ultimately, the nitrogen liquefies at approximately 77 degrees K. Continued cooling toward absolute zero tends to degrade the signal-to-noise ratio of the detector so that detector operation reaches an optimum working range of between 80 degrees K. and 65 degrees K. Liquid nitrogen forms around the cold finger and provides a thermally resistive coupling between the cold finger, which typically may be operating at a temperature of approximately 24 degrees K., and the temperature sensitive infrared detector.
A cryoengine and/or heater is controlled in a closed loop in order to maintain the detector temperature within the optimum range. The heating and cooling of the cold finger through closed loop control is seriously effected by the nonlinear thermal resistance of the liquid nitrogen which forms between the cold finger and the detector. This nonlinearity is due in large part to the heat of vaporization of the nitrogen as it boils and recondenses. The boiling, bubbling and thermal nonlinearity of the liquid nitrogen coupling in the thermal circuit of the cold finger and the detector substantially and seriously interferes with the efficiency of the detector and degrades the signal-to-noise ratio. When this thermal nonlinearity begins to predominate, what occurs is known as thermo-flash or video-flashing which is manifested as large erratic swings in the signal generated by the detector.
In order to avoid video-flashing, the prior art has sought various means to exclude or control the liquid nitrogen which inherently forms between the cold finger and the dewar. A typical method for such control is wrapping the cold finger with tape or other thermally resistive solid material in order to minimize the liquid nitrogen from the interstitial space between the cold finger and the dewar. The difficulty and limitation of this solution is the requirement for extremely tight tolerances between the cold finger and the glass bore of the detector if the tape is to uniformly fill the interlying void. Microscopic gaps are invariably formed along the length of the cold finger and dewar permitting the liquefication of nitrogen and consequent nonlinearity of the thermal resistance between the cold finger and the detector.
In order to avoid the microscopic gaps which are inherently formed between the cold finger and the dewar when any solid material was used as the filler, designs have been derived to fill the void between the dewar and cold finger with greases and gels. However, every compound, which has been used as a filler, has been characterized by physical or thermal properties which significantly change as cryogenic temperatures are approached. Typically, the grease or gel freezes solid and contracts. The material adheres at points inside the dewar to the inside glass surface with a bond strength exceeding the internal bond strength of the glass. Therefore, as the compound freezes and contracts, small portions of the glass are pulled away from the inside surface of the dewar, and a plurality of small pits are formed inside the dewar surface. Continued temperature cycling leads to catastrophic degradation or breakage of the dewar when this occurs.
Therefore, some means is needed whereby video-flashing in a cryogenically cooled detector assembly can be avoided by means compatible with continued temperature cycling between ambient to cryogenic temperatures.